1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO2, SO2, CO2, and H2O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as H2, CO, and hydrocarbon (CnHm). Preferably, the present invention relates to a gas sensor for measuring NO and NO2.
2. Description of the Related Art
Exhaust gas, which is discharged from vehicles or automobiles such as gasoline-fueled automobiles and diesel powered automobiles, contains nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO2), as well as carbon monoxide (CO), carbon dioxide (CO2), water (H2O), hydrocarbon (CnHm), hydrogen (H2), oxygen (O2) and so on. In such exhaust gas, about 80% of the entire NOx is occupied by NO, and about 95% of the entire NOx is occupied by NO and NO2.
The three way catalyst, which is used to clean HC, CO, and NOx contained in the exhaust gas, exhibits its maximum cleaning efficiency in the vicinity of the theoretical air fuel ratio (A/F=14.6). If A/F is controlled to be not less than 16, the amount of produced NOx is decreased. However, the cleaning efficiency of the catalyst is lowered, and consequently the amount of discharged NOx is apt to increase.
Recently, in order to effectively utilize fossil fuel and avoid global warming, the market demand increases, for example, in that the discharge amount of CO2 should be suppressed. In order to respond to such a demand, it becomes more necessary to improve the fuel efficiency. In response to such a demand, for example, the lean burn engine and the catalyst for cleaning NOx are being researched. Especially, the need for a NOx sensor increases.
A conventional NOx analyzer has been hitherto known in order to detect NOx as described above. The conventional NOx analyzer is operated to measure a characteristic inherent in NOx, based on the use of chemical luminous analysis. However, the conventional NOx analyzer is inconvenient in that the instrument itself is extremely large and expensive. The conventional NOx analyzer requires frequent maintenance because optical parts are used to detect NOx. Further, when the conventional NOx analyzer is used, any sampling operation should be performed for measurement of NOx, wherein it is impossible to directly insert a detecting element itself into a fluid. Therefore, the conventional NOx analyzer is not suitable for analyzing transient phenomena such as those occur in the exhaust gas discharged from an automobile, in which the condition frequently varies.
In order to dissolve the inconveniences as described above, there has been suggested a sensor for measuring a desired gas component in exhaust gas by using a substrate composed of an oxygen ion-conductive solid electrolyte.
FIG. 10 shows a cross-sectional arrangement of a gas analyzer disclosed in International Publication WO 95/30146. This apparatus comprises a first chamber 4 for introducing a measurement gas containing NO through a small hole 2 thereinto, and a second chamber 8 for introducing the measurement gas from the first chamber 4 through a small hole 6. Wall surfaces for constructing the first chamber 4 and the second chamber 8 are composed of zirconia (ZrO2) partition walls 10a, 10b through which oxygen ion is transmittable. A pair of measuring electrodes 12a, 12b, 14a, 14b for detecting the partial pressure of oxygen in the respective chambers are disposed on one of the ZrO2 partition walls 10a of the first chamber 4 and the second chamber 8 respectively. Pumping electrodes 16a, 16b, 18a, 18b for pumping out O2 in the respective chambers to the outside of the chambers are disposed on the other ZrO2 partition wall 10b respectively.
In the gas analyzer constructed as described above, the partial pressure of oxygen contained in the measurement gas G introduced into the first chamber 4 via the small hole 2 is detected by a voltmeter 20 as a difference in electric potential generated between the measuring electrodes 12a, 12b. A voltage in a range of 100 to 200 mV is applied between the pumping electrodes 16a, 16b by the aid of a power source 22 so that the difference in electric potential has a predetermined value. Accordingly, O2 in the first chamber 4 is pumped out to the outside of the apparatus. The amount of oxygen pumped out as described above can be measured by using an ammeter 24.
On the other hand, the measurement gas G, from which almost all of O2 has been removed, is introduced into the second chamber 8 via the small hole 6. In the second chamber 8, a difference in electric potential, which is generated between the measuring electrodes 14a, 14b, is detected by using a voltmeter 26. Thus, the partial pressure of oxygen in the second chamber 8 is measured. Further, NO contained in the measurement gas G introduced into the second chamber 8 is decomposed as follows by the aid of the voltage applied between the pumping electrodes 18a, 18b by means of a power source 28:
NOxe2x86x92(xc2xd)N2+(xc2xd)O2
O2 is generated during this process, which is pumped out to the outside of the chamber by the aid of the pumping electrodes 18a, 18b. At this time, a generated current value is detected by using an ammeter 30. Thus, the concentration of NO contained in the measurement gas G is measured.
In the case of the gas analyzer constructed as described above, the partial pressure of oxygen in the chamber is adjusted by measuring the minute voltage between the measuring electrodes 12a, 12b and between the measuring electrodes 14a, 14b, and the concentration of NO contained in the measurement gas G is measured by measuring the minute current between the pumping electrodes 18a, 18b. In this case, in order to maintain the measurement accuracy in the gas analyzer, it is necessary to sufficiently ensure the insulation performance between lead wires connected to the respective measuring electrodes 12a, 12b, 14a, 14b and the pumping electrodes 18a, 18b so that the variation in detection signal due to cross talk and disturbance is avoided as less as possible.
In general, the insulation performance between the lead wires is ensured in accordance with such methods as disclosed, for example, in Japanese Patent Publication Nos. 4-26055 and 5-62297, in which a porous insulative material is used to make insulation between the pumping cell and the sensor cell or make insulation between electrode lead wires. Those generally used as the material for ensuring the insulation performance as described above include alumina and spinel.
Further, in order to improve the pumping ability or improve the response performance when the electromotive force is measured, the respective electrodes used for the gas analyzer are produced by using porous materials. FIG. 11 shows an illustrative pattern of an electrode lead wire 34 which is wired from a through-hole 32 connected to an external connector to the measuring electrode 14b. In the illustrative arrangement shown in FIG. 11, porous insulative layers 36a, 36b are formed over and under the electrode lead wire 34 respectively to make insulation from other lead wires.
However, in the case of the conventional gas analyzer, the porous insulative layers 36a, 36b are formed to extend up to the through-hole 32. For this reason, a problem arises in that O2, which makes invasion from the outside through the through-hole 32, invades the second chamber 8 through the insulative layers 36a, 36b, and it increases the oxygen concentration in the vicinity of the measuring electrode 14b disposed near to the insulative layers 36a, 36b. 
Further, the electrode lead wire 34 is composed of a porous material. For this reason, a problem arises in that O2 invades the second chamber 8 through the electrode lead wire 34 from the connector side of the electrode lead wire 34 which is exposed to the outside through the through-hole 32, and it increases the oxygen concentration in the vicinity of the connecting section of the measuring electrode 14b with respect to the electrode lead wire 34. Especially, the measuring electrode 14b for the second chamber 8 tends to be affected by O2 having made the invasion. Therefore, an inconvenience arises in that the O2 increases the NO decomposition current.
Usually, a porous electrode composed of Pt is used for the measuring electrode 14b disposed at the inside of the second chamber 8. However, the use of such an electrode involves the following problem. That is, O2 gas is accumulated in the electrode lead wire 34 through the measuring electrode 14b, and the oxygen concentration in the vicinity of the measuring electrode 14b is increased upon the next pumping operation due to leakage of O2 from the electrode lead wire 34.
When the oxygen concentration in the vicinity of the measuring electrode 14b is increased due to the invasion of O2 into the second chamber 8 through the insulative layers 36a, 36b and the electrode lead wire 34 and due to the accumulation and leakage of O2 from the electrode lead wire 34 as described above, then an inconvenience arises in that the pumping current, which would otherwise depend on the decomposition of NO, is increased, and it becomes impossible to measure NO highly accurately.
The present invention has been made in order to overcome the inconveniences described above, an object of which is to provide a gas sensor which makes it possible to avoid invasion of oxygen through any route except for an introducing port for a measurement gas so that the amount of oxide or inflammable gas contained in the measurement gas may be measured extremely highly accurately.
According to the present invention, there is provided a gas sensor comprising a main pumping means including an inner pumping electrode and an outer pumping electrode arranged on inner and outer surfaces of a substrate composed of an oxygen ion-conductive solid electrolyte, for pumping-processing oxygen contained in a measurement gas introduced from external space on the basis of a control voltage applied between the inner pumping electrode and the outer pumping electrode; an electric signal-generating conversion means including an inner detecting electrode and an outer detecting electrode arranged on inner and outer surfaces of a substrate composed of an oxygen ion-conductive solid electrolyte, for decomposing a predetermined gas component contained in the measurement gas after being pumping-processed by the main pumping means, by means of a catalytic action and/or electrolysis to make conversion into an electric signal corresponding to an amount of oxygen produced by the decomposition; and insulative layers and conductive layers formed on a plurality of solid electrolyte green sheets, the plurality of green sheets being stacked and integrated into one unit followed by being sintered; wherein at least a lead wire connected to the inner detecting electrode of the electric signal-generating conversion means, which is exposed to the measurement gas, is densified; and the predetermined gas component contained in the measurement gas is measured on the basis of the electric signal detected by the electric signal-generating conversion means.
According to the present invention, at first, the oxygen, which is contained in the measurement gas introduced from the external space, is pumping-processed by the main pumping means, and the oxygen is adjusted to have a predetermined concentration. The measurement gas, which has been adjusted for the concentration of oxygen by means of the main pumping means, is introduced into the electric signal-generating conversion means in the next step. The electric signal-generating conversion means decomposes the predetermined gas component contained in the measurement gas after being pumping-processed by the main pumping means, by means of the catalytic action and/or electrolysis to make conversion into the electric signal corresponding to the amount of oxygen produced by the decomposition. Thus, the predetermined gas component contained in the measurement gas is measured on the basis of the electric signal supplied from the electric signal-generating conversion means.
When the electric signal-generating conversion means comprises a measuring pumping means and a current-detecting means, the measurement gas, which has been adjusted for the oxygen concentration by means of the main pumping means, is introduced into the measuring pumping means.
The measuring pumping means decomposes the predetermined gas component contained in the introduced measurement gas in accordance with the catalytic action and/or electrolysis. The oxygen produced by the decomposition is pumping-processed on the basis of a measuring pumping voltage applied between the inner detecting electrode and the outer detecting electrode. The pumping current, which is generated in the measuring pumping means corresponding to the amount of oxygen pumping-processed by the measuring pumping means, is detected by the current-detecting means. Thus, the predetermined gas component is measured depending on the amount of oxygen.
Alternatively, when the electric signal-generating conversion means comprises a concentration-detecting means and a voltage-detecting means, the measurement gas, which has been adjusted for the oxygen concentration by the main pumping means, is introduced into the concentration-detecting means. The concentration-detecting means decomposes the predetermined gas component contained in the introduced measurement gas in accordance with the catalytic action. An electromotive force of the oxygen concentration cell is generated depending on a difference between the amount of oxygen produced by the decomposition and the amount of oxygen contained in a gas existing on the side of the outer detecting electrode. The electromotive force is detected by the voltage-detecting means. Thus, the predetermined gas component is measured depending on the amount of oxygen.
In the present invention, at least the lead wire, which is connected to the inner detecting electrode of the electric signal-generating conversion means (the inner detecting electrode of the measuring pumping means or the inner detecting electrode of the concentration-detecting means) exposed to the measurement gas, is densified. Accordingly, the gas sensor is prevented from invasion of unnecessary oxygen from the outside through the lead wire. As a result, the amount of the predetermined gas component can be measured highly accurately on the basis of only the oxygen obtained from the predetermined gas component.
In the gas sensor according to the present invention, the lead wire may be composed of a cermet comprising a ceramic and a metal of the platinum group. In this embodiment, it is preferable that the ceramic contained in the lead wire has a sintering degree which is equivalent to or not less than a sintering degree of the solid electrolyte substrate.
Especially, when the lead wire is composed of a cermet comprising ZrO2 and a metal of the platinum group, it is preferable that ZrO2 contained in the lead wire has a sintering degree which is equivalent to or not less than a sintering degree of ZrO2 contained in the solid electrolyte substrate.
It is preferable that the lead wire has a porosity of not more than 10%. Further, it is preferable that the lead wire is in an insulated state which is maintained by using a densified insulative material.
The gas sensor according to the present invention may further comprise an auxiliary pumping means including an inner auxiliary electrode and an outer auxiliary electrode arranged on the inner and outer surfaces of the substrate composed of the oxygen ion-conductive solid electrolyte, for pumping-processing oxygen contained in the measurement gas after being pumping-processed by the main pumping means on the basis of an auxiliary pumping voltage applied between the inner auxiliary electrode and the outer auxiliary electrode.
Accordingly, the measurement gas, which has been firstly subjected to coarse adjustment for the predetermined gas component to have a predetermined concentration by the aid of the main pumping means, is further subjected to fine adjustment for the concentration of the predetermined gas component by the aid of the auxiliary pumping means.
In general, when the concentration of the predetermined gas component in the measurement gas in the external space is greatly changed (for example, when oxygen is changed from 0% to 20%), then the distribution of the concentration of the predetermined gas component in the measurement gas to be introduced into the main pumping means is greatly changed, and the amount of the predetermined gas component to be introduced into the measuring pumping means or the concentration-detecting means is also changed.
During this process, the oxygen concentration in the measurement gas after being pumping-processed by the main pumping means is finely adjusted in accordance with the pumping process effected by the auxiliary pumping means. However, owing to the pumping process performed by the main pumping means, the change in concentration of oxygen in the measurement gas introduced into the auxiliary pumping means is greatly reduced as compared with the change in concentration of oxygen in the measurement gas introduced from the external space (measurement gas introduced into the main pumping means). Accordingly, it is possible to accurately and constantly control the concentration of the predetermined gas component in the vicinity of the inner detecting electrode of the measuring pumping means or in the vicinity of the outer detecting electrode of the concentration-detecting means.
Therefore, the concentration of the predetermined gas component introduced into the measuring pumping means or the concentration-detecting means is scarcely affected by the change in concentration of oxygen in the measurement gas (measurement gas introduced into the main pumping means). As a result, the pumping current value detected by the current-detecting means or the electromotive force detected by the voltage-detecting means is not affected by the change in oxygen concentration in the measurement gas, which has a value accurately corresponding to the amount of the objective component existing in the measurement gas.
It is preferable to densify the lead wire and/or the insulative layer concerning the inner auxiliary pumping electrode, for the purpose of accurate control of the oxygen concentration in the measurement gas.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.